On Mon, Nov 24, 2008 at 12:39 AM, Niklas Broberg
I want this information to be used somehow when creating the Template Haskell AST, so that the operators used have the correct fixities. If I use HSE for parsing Haskell expressions, then I want it to tell me where it unsure of the fixities, so that I can insert the correct ones there. For this use case, I would want HSE to use an AST like I suggested, because it allows the parser to say, "I'm not sure what the correct fixity is".
As noted above, I really don't like that change. If you use HSE for parsing expressions, it would *never* know the correct fixities, and you would always get a completely left-biased tree. Why would that be harder to work with? I understand the argument about least surprise, and that this feature must be strongly documented (which it currently isn't), but for practical purposes I don't see why the current state would be problematic. It should even be trivial to convert the left-biased tree into an intermediate structure exactly like the one you suggest, no?
No, I believe it wouldn't. The left-biased tree cannot distinguish where parentheses have been used from where HSE inserted its own left fixities. For instance, if we have the expressions xs ++ ys ++ zs (xs ++ ys) ++ zs Then HSE will return something like (I'm using strings for the subexpression parses, for simplicity) InfixE (InfixE "xs" "++" "ys") "++" "zs" for both the first and second parses. However, if I then use the knowledge that ++ is right infix, I will want to convert the first, but not the second parses into right infix. I can't do this, because they are both parsed the same way. I would also like to point out that a list representation as I suggested can in fact encode the correct fixities if they are known to HSE. This is true simply because the list constructor is isomorphic to the current constructor in the special case where the list of operators has length 1. For instance, in the first example above, if HSE somehow knew that ++ is right infix, it should return a parse result of InfixE ["xs", InfixE ["ys", "zs"] ["++"]] ["++"] rather than just InfixE ["xs", "ys", "zs"] ["++", "++"] Cheers, Reiner
Cheers,
/Niklas